PRINCIPLES OF LOW VISION AIDS

Abdelmonem M. Hamed, M.D. Professor of ophthalmologyBenha College of MedicineFellow of Baylor College, Texas Medical Center, Houston, USA

Definition of subnormal vision Legal blindness:

An acuity rating in which BCVA achieves no more than 3/60 for distance in the better eye.

Or, a defect in visual field in which the widest diameter of vision subtends an angle no greater than 20o

Definition of subnormal vision Low vision Pt.:

Is a person who has the following: acuity of < 6/18 to 3/60

in the better eye, from all causes.

visual field response bellow normal

The VA can not be corrected with spectacles

Aim of low vision aids

to enhance vision-related quality of life for people with functional low vision

Causes of low vision

Corneal opacity, dystrophy

Sub-luxation Senile macular

degeneration. Optic atrophy Etc.

Types of low vision aids: Optical visual aids:

Contact lenses Magnifying glasses Pin-hole spectacles Typo-scope High plus reading

lenses Telescopic lenses Prismatic lenses Projection devices

Types of low vision aids:

Non-optical visual aids: Large printing books Large sized pens Talking books Line guide Illumination Reading stands filters

Angular magnification

N = the near point of the eye = 25 cm 

The maximum magnification is achieved by bringing the convex lens right up to your eye and then arranging the lens, object and your head so that the image is at the near point.

N

Application of angular magnification

Angular magnification

Tan 01= O/25

O01

25CM

Rule No. 1: objects viewed at near point of unaided eye (25cm) subtend angle (01) at the eye

Angular magnification

Tan 02 = O/f

O 02

20D lens

f

f1

Rule No.2: Objects viewed close to the eye through a convex lens, (with objects at first principle focus of convex lens), objects and image subtend angle 02 at the eye.

I

f2

Angular magnification

If M= tan02/tan01 = (O/f) / (O/25) So, M= O/f X 25/O So, M = 25cm/f Where f is the focal distance of the

convex lens (f = 100/power in diopter) in cm.

So, f of 20 D lens = 100/20= 5cm What is the magnification of 20 D

lens?M= 25/f = 25/5 = 5X

Factors that increase angular magnification If the object lies directly at F1 of the

lens (maximum magnification) If 2nd convex lens is placed between

object (which is brought closure to the eye) and the eye. Total angular magnification= M1 + M2

Calculation of required magnification

Near point magnification Formula= print size in

meter/distance from eye to chart in centimeter = diopter of add needed

Ex.: If Pt. read 2M at 40 cm = 200cm/40cm = 5 D of add needed to read M1(rough method) (as a starting point)

Calculation of required magnification Near point magnification

Ex2: IF Pt read 4M at 10cm = 400 cm / 10 cm = 40 D of add needed to read M1

To convert diopter to X magnification, divide the diopter/4 Ex.: 40D = 40/4 = 10 X

Calculation of required magnification Distance vision magnification:

Follow the following formula: the Pt. real

distance VA / desired VA = X magnification

Ex.: If real Pt. VA = 20/200 And desired VA = 20/50 SO, 200/50 = 4 X magnification (rough

method) (as a starting point)

Practical points

Using the contact lenses as low vision aids: To correct irregular astigmatism (hard

CL) Telescopic like action: positive glass

lenses with minus CL Reversed telescope: minus glass lenses

with positive CL Pine hole CL like in: coloboma of iris,

aniridia, etc.

Practical points

Magnifiers Hand held magnifiers Stand magnifiers With or without

illumination Pin hole spectacles:

to improve VA in Pt. with irregular astigmatism with good macular function

Typo-scope By enhancing the

contrast

Can u see now

Practical points

High plus reading glasses Power from + 4 to + 40 D Focal distance?

Practical points Telescopes

Design: Galilean Ts = high minus ocular + high plus

objective (erect image) Keplerian Ts = plus ocular and objective

lenses (inverted image). It uses prism to reinvert the image.

Practical points Telescopes

Wight: Galilean is lighter than Keplerian Ts

Field of vision: Increasing the power in either design will

decrease the field

Ts 5X Ts 2X

Practical points Telescopes

Power: the spectacle mounted

Galilean Ts are relatively useful up to 4 X

beyond 4 X the field is very small and the light gathering ability

decreases The Kalerian is usually

preferred by Pts. At and above 4X

Practical points Telescopes

Focus ability: near, intermediate, and distance

Mounting position: Ocular surface Across the bridge

(monocular vision)

Practical points

Telescopes Intraocular Ts

Low vision enhancement system

Practical points CCTV

Closed circuit TV Advantages:

Greater brightness Improved contrast Reduced aberration and distortion Longer reading distance Greater magnification ( up to 60 times)

It is not the 1st aid, it is used when: Field restricted <5 degrees A higher magnification needed

Treatment of central scotoma Causes:

CNVM Chorio-retinitis Diabetic retinopathy Macular hole Etc.

Treatment of central scotoma The suspected VA as we go

away from fovea: VA at fovea = 6/6 VA at 2.5 degrees from fovea =

6/12 VA at 5 degrees from fovea = 6/12

– 6/24 VA at 10 degrees from fovea = 6/24

Treatment: Ts, Magnifiers, CCTV, prism (move

the image towards the functioning retina), non optical aids like large sized prints, filters to enhance contrast.

fovea

How to prescribe a prismatic glass Refract Start with the better eye Start with 10 ∆ diopter lens from the trial set Put the base at 90o in the trial frame and test VA Move the base of the prism at 45o interval around the

clock and test the VA in each position (at 2 meter or less)

Note axis of best acuity Introduce identical power prism for the fellow eye Test the VA while both eyes open and ask Pt. if he feels

comfort with the glass Ex. OD -1.5 -1.25 ax 90 pr 10 ∆ base up and out at 135o

OS -2.0 -1.00 ax 95 pr 10∆ base up and in at 120o

Treatment of narrow field of vision

Causes: RP Glaucoma Proliferative

diabetic retinopathy

Malignant myopia Etc.

Treatment of narrow field of vision Treatment

Use the optical aid that increase field of vision: Inverted Ts Concave lenses e.g. -8 to -12 D

They increase the field but decrease the VA

Mobility aids like dogs, etc.

Management of nystagmus

Base out prism spectacles to stimulate

convergence dampens the

nystagmus2mm 2mm

Management of nystagmus

How to fit the fresnel prism? Leave 2 mm from the

visual axis for scanning by the Pt.

Ask the Pt. to come back after 2 weeks, then adjust the apex of the prism…..move it far than 2mm, until reaching the Wright position

2mm

3mm